The present invention relates to an ignition device for an internal combustion engine having multiple cylinders and direct gasoline injection, at least one ignition coil being provided for each cylinder, the primary side of the ignition coil being switched by an ignition switch controlled by a microprocessor and a spark plug being connected to the secondary side of the ignition coil.
In direct gasoline injection, gasoline is injected into the combustion chamber of a cylinder, where it is evaporated and ignited by the secondary high voltage of the ignition coil. If the secondary current is cut off too soon, uncombusted or partially combusted gas may escape. To guarantee reliable operation with low exhaust emissions, several ignition sparks, for example, can be produced by double coil ignition or pulse train ignition. In addition, the secondary current can be prolonged.
In principle, the duration of the secondary current can be prolonged by increasing the primary current in the ignition coil, because this increases the energy transferred to the secondary side. Such an energy increase, however, is counteracted by the coil saturation that occurs with an increase in the primary current and the increasing power losses in the ignition coil, preventing an effective increase in the secondary current and its duration. In addition, the ignition output stage and the ignition coil may be overloaded thermally by high switching currents. Therefore, this measure for prolonging the duration of the secondary current should be limited only to those operating states in which it is absolutely necessary, such as a cold start, to avoid unnecessary bum-up of the spark plugs. In all other operating states, it should be possible to switch back to the xe2x80x9cnaturalxe2x80x9d secondary current conditions.
The present invention provides an ignition device for an internal combustion engine with which the secondary current conduction time of the ignition coil can be prolonged controllably without increasing the primary current.
This is achieved according to the present invention by applying an external voltage to the ignition coil to prolong the secondary current conduction time.
The present invention is based on the recognition of the fact that the secondary current conduction time can be prolonged if an external voltage which supplies the power required for the prolonged secondary current is applied at the primary side or at the secondary side of the ignition coil.
Although it is possible in principle to supply an external voltage on the secondary side of the ignition coil, this is difficult because of the high voltage (30 kV) occurring on the secondary side, so that the external voltage can advantageously be applied to the primary side of the ignition coil.
There are essentially various options for implementation of the ignition device according to the present invention.
In a first advantageous variant of the present invention, the secondary current in the ignition coil is prolonged by controlled switching on and switching off of an auxiliary voltage source on the primary side. In this variant, the starter hardware known from practice can be used without conversion. In the future, a 14-volt voltage source operated via the ignition coil as well as a 42-volt voltage source will be available in motor vehicles, the latter then being available for use as an auxiliary voltage source to advantage.
In a second advantageous variant of the present invention, the secondary current in the ignition coil is prolonged with the help of the cut-off voltage of an auxiliary circuit having an auxiliary switch and an external inductor. The auxiliary transistor is switched off shortly before the end of the xe2x80x9cnaturalxe2x80x9d secondary current. This variant requires a second inductor and under certain circumstances also requires redesign of the ignition coil.
A third variant of the present invention makes use of the fact that prolonging the combustion time in direct gasoline injection is usually the goal in the case of single-spark coils, where an attached or external ignition switch and a rod coil can always be allocated to one cylinder of the engine. In this case, there are several inactive coil-ignition switch combinations for each active coil-ignition switch combination at any given moment, so that in the case of engines having an even number of cylinders, an inactive coil-ignition switch combination can be associated with each disconnecting coil-ignition switch combination. It is also conceivable to have an association such as that in the case of double coil ignition, where a parasitic spark is ignited in the exhaust. In the case of the ignition device according to the present invention, however, no ignition sparks should be produced by the passive coil-ignition switch combination. The passive coil-ignition switch combination should contribute only to prolonging combustion time. It is important that once the association of coil-ignition switch combinations has been made, it is reversible. In other words, when one coil-ignition switch combination generates an ignition spark, the associated coil-ignition switch combination serves only to prolong the combustion time and vice versa.